E-fuse (“electronic fuse”) circuits control a connection between an input voltage supply and a load coupled at an output terminal. The E-fuse can include a series power transistor that connects a load to an input power supply. For example, a circuit board may get its power from a bus. When the circuit board is inserted into the bus socket, contacts in the bus socket connect the board to power. E-fuses often provide: current over-limit controls; short-circuit protection; in-rush current limiting; dv/dt or start-up voltage ramp control; and reverse current protection. An E-fuse can reduce the available current to the load or even shut down the power supply connection to the load entirely when an overcurrent exists.
In an example application, a power transistor has a drain terminal coupled to a voltage supply and a source terminal coupled to a load at an output terminal. When providing power to the load at the output terminal, the gate of the power transistor needs to be at a sufficient voltage to turn on the power transistor to couple the load to the power supply. The current to the load is monitored using sensing circuitry. If the current flowing through the series power transistor exceeds a current limit, the gate voltage of the power transistor can be reduced to limit load current, or the gate voltage can be changed to cut off the power transistor. The turn off needs to happen before any physical damage to the power transistor can occur. The load current can exceed a current limit if a short to ground occurs at the output terminal or if a short occurs in a load circuit.